This invention relates generally to ring laser gyroscopes (gyros) and more particularly to ring laser gyros which are dithered to reduce the effects of mode locking otherwise referred to as "lock-in."
Ring laser gyroscopes feature a pair of light beams which are counterpropagated around a closed loop path. The two counterpropagating light beams are compared in frequency. When the gyro rotates in inertial space, the two counterpropagating beams see apparently unequal paths. This causes the beams to resonate at two different frequencies. The frequency difference is measured to provide an indication of the direction and rate of rotation of the gyro.
For small rotational rates a phenomenon referred to as mode or frequency locking or "lock-in" occurs. The frequencies of the two counterpropagating light beams lock to each other and the beams resonate together. This creates a dead zone for which no useful gyro output is obtained.
Various arrangements have been employed to avoid or reduce the lock-in phenomenon. One such arrangement features mechanical dithering of the gyro. The entire gyro is mechanically oscillated at a relatively high frequency. This gives the gyro a dynamically varying bias rotation rate which exceeds the threshold input rate or natural lock-in rate of the dead zone. While a mechanical arrangement of the type described accomplishes the desired results, it adds undesirable energy consumption, cost, weight, size and complexity to a system, such as an attitude reference system, employing the gyro. Mechanical dithering also introduces an undesirable source of vibration and noise into the system which may cause problems in the use of other instruments such as accelerometers or the like which are mounted with the gyro in the system.
Optical devices have been inserted into the laser cavities to introduce a bias or asymmetry into the operation of the laser to avoid the aforenoted mode locking. However, a problem arises in the optical approach in that the optical elements increase the number of interfaces between dissimilar materials in the optical path. Each interface tends to be a source of increased light scattering so that the mode locking effect which the optical devices attempt to reduce is actually enhanced. Further, the optical elements themselves, which may include Faraday cells or other like devices, introduce bias errors into the operation of the gyro due to such factors as, for example, asymmetry and random variations in their characteristics due, for example, to temperature variations.
Electrical excitation, or pumping power, for lasing is typically applied to the laser gaseous medium from a direct current supply. A voltage sufficient to sustain a discharge in the medium is applied to electrodes arranged in spaced relation. The discharge, or plasma excitation current, includes a flow of ionized gas between oppositely polarized electrodes in the optical cavity. This plasma flow tends to induce a bias in a ring laser gyro. Therefore, ring laser gyros using direct current excitation typically include a balanced electrode structure wherein two electrodes of one polarity are symmetrically disposed about a single electrode of the opposite polarity. With the arrangement described, two equal and opposite plasma excitation currents are caused to flow and their biases are thereby caused to cancel. The electrical excitation to sustain the plasma excitation currents may be obtained from the supply which varies the voltage so as to dynamically unbalance the currents. A dynamically varying bias in a gyro output, i.e. a dither, may thereby be induced. However, the maximum amount of dither obtainable in this way is too small to be given any practical consideration.
Other dithering schemes have been attempted. One such scheme involves pumping the plasma more vigorously. A linear induction motor is positioned adjacent the laser gain section and pumps the ionized gas unidirectionally. The apparent index of refraction for the counterpropagating light beams is therefore made different by an amount sufficient to avoid mode locking. This arrangement uses large amounts of energy and the requirement for an electromagnetic structure increases the weight of the system. Additionally, in a scheme such as described where a unidirectional bias is induced, no cancellation of systematic errors occurs.
U.S. Pat. No. 4,325,033 issued to Shutt on Apr. 13, 1982 and assigned to Rockwell International Corporation attempts to avoid the disadvantages of the aforenoted dithering schemes. The patented arrangement features a mechanically dithered ring laser gyro in which the effects of mode locking at low rotation rates are reduced by providing an oscillatory flow of the ionized gas in the gain section of the laser. The structure involved features an envelope in which the laser is sealed and includes a pair of closed-end bellows. The bellows are extended and retracted in push-pull relationship to pump the gas so as to create a dynamically varying pressure differential and hence oscillatory gas flow along the bore of the gain section of the laser which induces a corresponding oscillatory bias in the gyro output. Here again, the bellows arrangement adds weight and size to a system using a ring laser gyro and is prone to intolerable inaccuracies for many applications.
The present invention avoids the aforenoted problems by pneumatically establishing an oscillatory motion directly to a segment of the excited gas which serves as the photon source. Alternately switching ionizing potential between two cathodes in appropriately designed cavities establishes the desired oscillatory gas velocities for effecting the emitted frequencies within the gyro thereby permitting the detection of rotational rates below the "lock-in rate" of the otherwise undithered gyro.